Poly(methyl methacrylate) blends with quaternized polyethyleneimine: A study of morphology, optical, thermal and mechanical properties

Abstract

Abstract. This study aimed first to promote the alkylation of polyethyleneimine (PEI), developing its alkylated (quaternary) form (QA-PEI) by inserting alkyl groups into amine groups. Subsequently, polymer blends with poly(methyl methacrylate) (PMMA) were prepared via solvent casting, and finally, the physicochemical, optical, and mechanical behavior of the resulting PMMA/QA-PEI were assessed. Elemental analyses, Fourier-transform infrared spectroscopy (FT-IR), and hydrogen nuclear magnetic resonance spectroscopy ('H-NMR) confirmed that the PEI alkylation successfully converted the amine groups into quaternary ammonium groups. When added to PMMA, QA-PEI altered its coloration, making it yellow. In addition, higher contents of QA-PEI hindered PMMA transmittance and increased its opacity due to the larger QA-PEI domains. Scanning electron microscopy (SEM) images showed that PMMA and QA-PEI formed a phase-separated system, establishing a droplet-matrix morphology. The thermal and mechanical behavior showed some compatibility between PMMA and QA-PEI as thermal resistance slightly improved and PMMA glass transition temperature (Tg) decreased. The tensile strength was also improved in the PMMA/QA-PEI blends without significant change in strain at break and tensile modulus.

Headnote

Abstract. This study aimed first to promote the alkylation of polyethyleneimine (PEI), developing its alkylated (quaternary) form (QA-PEI) by inserting alkyl groups into amine groups. Subsequently, polymer blends with poly(methyl methacrylate) (PMMA) were prepared via solvent casting, and finally, the physicochemical, optical, and mechanical behavior of the resulting PMMA/QA-PEI were assessed. Elemental analyses, Fourier-transform infrared spectroscopy (FT-IR), and hydrogen nuclear magnetic resonance spectroscopy ('H-NMR) confirmed that the PEI alkylation successfully converted the amine groups into quaternary ammonium groups. When added to PMMA, QA-PEI altered its coloration, making it yellow. In addition, higher contents of QA-PEI hindered PMMA transmittance and increased its opacity due to the larger QA-PEI domains. Scanning electron microscopy (SEM) images showed that PMMA and QA-PEI formed a phase-separated system, establishing a droplet-matrix morphology. The thermal and mechanical behavior showed some compatibility between PMMA and QA-PEI as thermal resistance slightly improved and PMMA glass transition temperature (Tg) decreased. The tensile strength was also improved in the PMMA/QA-PEI blends without significant change in strain at break and tensile modulus.

Keywords: polymer blends, morphology, polymer films, functional polymer, surface property, alkylation

1. Introduction

Cationic polymers, also called polycations, are polymers that contain positive charges in their side chains or along their main chains' backbone [1]. These positive charges arise from specific chemical groups present in a polymer, such as quaternary amine or phosphine groups, which yield quaternary ammonium or phosphonium groups, whose positive charges derive from nitrogen or phosphorus atoms that arc covalently bound to four other ligand atoms [1,2]. Applications of this type of polymer have been extensively studied and consist mainly of improvements on drug delivery systems by encapsulating bioactivc molecules, increasing their solubility, retention time, and stability, preventing immediate degradation, and enhancing absorption into the desired tissue [1, 3-5]. These properties render cationic polymers suitable as non-viral gene transfection agents, delivering bioactive molecules such as nucleic acids and proteins [6, 7]. Other applications of this kind of polymer are based on developing antimicrobial materials since some cationic polymers can inactivate a broad spectrum of pathogenic microorganisms with high efficiency [8-13]. Cationic polymers are often used on surfaces due to their antifouling properties, especially against solute ions or microbial biofilm formation [14, 15]. For purifying wastewater contaminated with heavy metal particles and ions, cationic polymers are often used as antifouling agents on separation membranes, thus increasing the purification efficiency [16, 17].

Polyethyleneimine (PEI) is a polymer that can be classified as a polyamine [8-10], whose repeating unit consists of two saturated carbons followed by an amine group. It exists in linear and branched forms, and in the branched form of PEI, the amine groups can be primary, secondary, or tertiary [10,13]. These nitrogen atoms in PEI can be converted to an alkylated (quaternary) form, usually named quatcrnized polyethyleneimine (QA-PEI), by inserting alkyl groups into these amine groups. This converts them into naturally positively charged ammonium groups, rendering this polymer to be permanently positive-charged [10, 12, 13, 17]. Many studies in the literature assessed different chemical routes for the PEI alkylation process [10-14, 19], applying QA-PEI for many other purposes. One of the most promising applications for QA-PEI is developing antimicrobial materials [8, 9, 11-14, 18, 20-22], yielding high efficiency in the inactivation of a broad spectrum of microorganisms. For instance, QA-PEI has already been applied as a nanoparticle in dental restorative resins [22-26].

QA-PEI has also been studied in drug delivery systems, where it is used for medication encapsulation [27-29] and as a non-viral gene transfection agent, also encapsulating and target-delivering bioactive molecules [30-32] and for water treatment purposes [33]. The branched configuration of PEI, when alkylated, yields a higher positive charge density when compared to the linear configuration, and this can enhance the properties mentioned above and performance [33-42]. Although QA-PEI has been extensively studied and deemed suitable for different applications, relatively few studies have been done on using quaternized PEI as a component of polymeric blends, aiming to modify the properties of a majority polymeric phase.

Poly(methyl methacrylate) (PMMA) is a highly suitable polymer for optical and surface applications, such as displays, screens, and surface coatings. It has remarkable properties, is relatively lightweight, and has low cost [43]. Therefore, it was chosen as the polymeric matrix in this study. A reason for developing PMMA blends with QA-PEI is that PMMA is extensively used in applications where biocompatibility is necessary, such as orthodontic implants or prostheses [44-48]. However, this polymer is highly susceptible to contamination by various microorganisms, which can lead to the development of bacterial biofilm on its surface [46^19], and blending PMMA with QA-PEI can help with that. Another typical application where PMMA is prone to contamination is touchscreen panels and displays for electronic devices [43] and other kinds of applications, such as membranes for filtering of contaminated water [37, 39, 40].

In this context, this study focuses on the quaternization of PEI and its influence as a modifier agent of PMMA. The method for mixing and developing PMMA/QA-PEI blends utilized solvent casting, a simple and cost-effective approach for creating polymer blends. Here, a PEI alkylation route was addressed, with subsequent use as a component in polymer blends with PMMA. Its influence on the optical, thermal, and physical-mechanical properties of PMMA was accessed.

2. Experimental section

2.1. Materials

For QA-PEI production, a 50% aqueous solution of branched PEI, Mn ~ 60 000 Da by gel permeation chromatography (GPC), under code 181978, was purchased from Merck®, Saint Louis, USA. The chemical structure of the branched PEI used in this study is shown in Figure 1. Absolute ethanol under code P. 10.0051.015.27.84, from Dinamica®, Indaiatuba, Brazil, was used as a solvent for the alkylation steps. For the first alkylation step of the PEI, 1bromohexane, 98% pure, under code B68240, from Merck®, Saint Louis, USA, was used. For the second PEI alkylation step, iodomethane, 99% pure, under code 67692, from Merck®, Saint Louis, USA, was used. Solid sodium bicarbonate (NaHCOg), 98% pure, from Exodo®, Sumarc, Brazil, under code BS06128RA, was used to neutralize the resulting acids and the alkylation steps. For the development of the PMMA/QA-PEI blends, a PMMA (Mn~ 120000 Da by GPC), under code 182265, was chosen as the matrix and chloroform as the solvent, both provided by Merck®, Saint Louis, United States of America.

2.2. Methods

2.2.1. PEI alkylation

This step promoted the alkylation of PEI, aiming to insert alkyl groups in the primary, secondary, and tertiary amine groups present in this polymer, thus converting these amine groups into quaternary ammonium groups, promoting the high density of positive charges. Using a methodology adapted from Park et al. [12], the aqueous solution of PEI was subjected to a lyophilization process (using a LI01 lyophilizer, from Liobras, Sao Carlos, Brazil) to remove water, followed by inserting 5.9 g of the lyophilized PEI into a 250 mL round bottom flask. Next, the PEI was solubilized in 150 mL of absolute ethanol at room temperature.

The schematic of the alkylation steps is shown in Figure 2. The insertion of hexyl groups into the amine groups of the PEI was promoted by adding 62.3 g of 1-bromohcxanc to the flask, which was then conditioned under reflux for 48 h at 95 °C. The acids were neutralized by adding 13.5 g of sodium bicarbonate. The flask containing the solution was kept under reflux conditions for an additional 48 h at 95 °C, under stirring, to ensure that neutralization was completed.

The second step starts with adding 37.0 g of iodomethane to the flask after the neutralization, aiming to insert methyl groups into the nitrogen atoms. Reflux conditions were maintained at 60 °C for 72 h. The acids produced as a by-product were neutralized by adding 10.8 g of sodium bicarbonate. After that, the sample was left under reflux conditions at 60 °C for another 72 h to ensure complete acid neutralization. The precipitates formed in both neutralization steps were separated by vacuum filtration and discarded. The supernatant was then inserted in an ionexchange column to remove the remaining halide ions in the solution. The excess solvent was evaporated from the solution and the remaining alkylated PEI (QA-PEI) was thoroughly washed, alternating deionized water and hexane. The QA-PEI obtained in this process was placed for drying in a circulating air oven at 45 °C for 24 h.

2.2.2. PMMA/QA-PEI development

PMMA/QA-PEI films were produced via solvent casting method using chloroform as a solvent. Five different samples were made, varying PMMA and QA-PEI concentrations in weight [wt%] on the films, according to Table 1. PMMA and QA-PEI were dissolved and homogenized for each sample under magnetic stirring at 1400 rpm for six hours at room temperature.

After solvent casting, the polymer blends were poured onto glass Petri dishes and left to dry at room temperature for 24 h, with subsequent drying at 45 °C in an air circulation oven. Finally, they were removed from the oven and conditioned again at room temperature for one hour before being stored on sealed polyethylene bags in a dark environment at room temperature.

2.3. Characterization

2.3.1. Elemental analyses

Elemental analyses were carried out on samples of lyophilized pristine PEI and neat QA-PEI to determine the carbon, hydrogen, and nitrogen content and assess the nitrogen quaternization levels achieved on the alkylation steps. These analyses were performed using a CHN2400 elemental analyzer from PerkinElmer Inc. (Norwalk, USA), following the PreglDumas method.

2.3.2. Hydrogen nuclear magnetic resonance (XH-NMR) analyses

^-NMR analyses were conducted on samples of PEI and QA-PEI to assess the chemical structural differences that the alkylation process caused, evaluating if the process successfully converted the amine groups on PEI into quaternary ammonium groups. NMR analyses were performed using a Bruker Corporation (Billerica, USA) Avance III 600 spectrometer, using deuterated water as solvent and tetramethylsilane as reference.

2.3.3. Fourier transform infrared spectroscopy

(FT-IR)

Fourier transform infrared spectroscopy (FT-IR) analyses were performed on lyophilized PEI and QA-PEI samples to verify the presence of the characteristic chemical groups on QA-PEI to evidence the alkylation. The analyses were performed using an Agilent Technologies, Inc. (Santa Clara, USA) Cary 630 infrared spectrometer, with the attenuated total reflectance (ATR) accessory, at 25 °C, with the following analysis conditions: Spectral window ranged from 4000 to 400 cm-1; resolution of 2 cm-1 number of sample scans: 128 readings.

2.3.4. Ultraviolet (UV)-visible transmittances

To evaluate the transmittances of the PMMA/QA-PEI blends films produced via solvent casting, measurements of radiation transmittance in the UV-visible region were performed using a Shimadzu Corporation (Kyoto, Japan) UV-1800 ultraviolet-visible spectrophotometer, with the following conditions: wavelengths: 200 to 800 nm; sampling interval: 0.2 nm.

2.3.5. Opacity

The opacity (OP) of the PMMA/QA-PEI blend films was measured using the UV-VIS transmittance analyses, applying Equation (1) [50] on the absorbance values at 600 nm wavelength:

OP = ^600nm(1) A

where Abs^Q nm is the absorbance at 600 nm, and x is the film thickness [mm].

2.3.6. Scanning electron microscopies (SEM)

Scanning electron microscopies (SEM) were performed on the cryo-fractured cross sections of the pristine PMMA, PMMA/2%QA-PEI, and PMMA/10%QA-PEI film samples using a FEI Company (Hillsboro, USA) Inspect S 50 microscope and imaging using secondary electrons at 5.0 kV voltage and 4.0 nm spot. The samples received a metallic coating in an EMITECH Ltd. (Montigny-le-Bretonncu, France) Sputtering coater machine, model K450, depositing a gold layer.

2.3.7. Contact-angle measurements

Contact-angle measurements were performed on the surfaces of the PMMA/QA-PEI films using the sessile drop technique with deionized water. The measurements were performed using a NanoScience Instruments (Pheonix, USA) Attension® Theta Lite TL101 optical tensiometer.

2.3.8. Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry analyses were performed on the lyophilized PEI and QA-PEI samples and on pristine PMMA and PMMA/QA-PEI blends films using a TA Instruments® (New Castle, USA) Q100 thermal analyzer, with the following analysis conditions: Sample mass of approximately 10 mg, temperature range: -60 to 250 °C, heating rate: 20 °C/min, inert atmosphere, with argon gas flow of 50 mL/min and alumina crucible.

2.3.9. Thermogravimetric analyses (TGA)

Thermogravimetric analyses were performed on the lyophilized PEI, QA-PEI, Pristine PMMA, and PMMA/QA-PEI blends films using a TA Instruments® (New Castle, USA) TGA 2950 thermogravimetric analyzer, with the following analysis conditions: Sample mass of approximately 10 mg, temperature range: 25 to 850 °C, heating rate: 10 °C/min, inert atmosphere, with nitrogen gas flow of 50 mL/min.The onset temperature (Tonset) was defined by extrapolating the lines of the weight loss steps on the TGA curves, and the crossing points of these extrapolations were defined as Tonset.

2.3.10. Mechanical properties

Tensile strength, strain at break, and tensile modulus results were obtained on ten rectangular specimens (80x15 mm) of PMMA/QA-PEI blends according to ASTM D882-18 using an împac (Vargem Grande Paulista, Brazil) IP-90COM universal testing machine with the following conditions: grips with the initial distance of 50 mm; test speed: 12.5 mm/s.

3. Results and discussion

3.1. Elemental analyses

The Elemental analyses with carbon (C), hydrogen (H) and nitrogen (N) contents and the carbon / nitrogen ratios (C/N) for each sample are displayed in Table 2.

Following a methodology from Yudo vin-Farber et al. [18], the values obtained for samples of PEI and QA-PEI should not be directly compared; hence, the C/N ratios were calculated. As seen in Table 2, the C/N ratio for QA-PEI is greater than that for PEI, meaning that the carbon content on the QA-PEI sample was relatively higher on this sample compared to PEI, which implies the alkylation process was effective in inserting the alkyl groups on the nitrogen atoms of PEI, thus converting them into quaternary nitrogen [18]. The obtained 5.88 ratio for QA-PEI is similar to the highest values reported in the literature [18,41,52].

As the PEI used in this study is a highly branched polymer with a high density of primary, secondary, and tertiary amine groups, very significant effects of steric hindrance arise from its branches [41^43] and, mainly because there are amine groups with different degrees of substitution, it is not possible to achieve high levels (above 80%) of alkylation of these amine groups using only 1-bromhexine on the first step [814,41^13].

3.2. Hydrogen nuclear magnetic resonance (1H-NMR) analyses

The 'H-NMR spectrum for the PEI and QA-PEI samples is shown in Figure 3 and Figure 4, respectively, along with their chemical structure with assigned carbon atoms.

From Figure 3, between 2.50 and 2.80 ppm, it is noticeable that there are chemical shifts related to the hydrogen atoms on the different methylene groups in the PEI chemical structure, assigned from a to e. The obtained spectrum is consistent with the chemical structure of the polymer and with what is previously reported in the literature [8], corroborating the results obtained by other characterization techniques and indicating that there are no contaminating compounds in this sample.

Figure 4 shows the 1H-NMR spectra for the QA-PEI sample. Around 0.77 ppm, there is a chemical shift referent to the hydrogen atoms on the terminal methyl groups of the inserted alkyl chains [8]. At around 1.23 ppm, there is a chemical shift related to hydrogen atoms of the methylene groups of the alkyl groups located 2 carbon atoms away from the nitrogen of the quatemized ammonium groups [8]. Around 1.68 ppm, there is a chemical shift referring to the hydrogen atoms of the carbon located at one carbon atom distance of the quaternary ammonium group [8]. Between 2.80 and 4.35 ppm, there are several overlapping peaks, referring to the hydrogens of carbon atoms from a to e, which are from the original PEI structure, and carbon atom , which is the carbon atom from the inserted alkyl chains that arc bound to the quaternary nitrogen atom [8]. These results elucidate the structure of QA-PEI with the alkyl groups inserted into the nitrogen atoms and prove the efficiency of the quaternization process.

3.3. Fourier transform infrared spectroscopy (FT-IR)

The FT-IR spectra of PEI and QA-PEI are shown in Figure 5. The FT-IR spectrum of PEI exhibits characteristics of an amine group's absorption band around 3267 cm-1 (N-H) and a band at 1595 cm-1 (N-H) [9, 18]. Other characteristic bands are shown on 2936, 2890, 2805 and 1454 cm'1 (C-H) [9, 18]. These results are similar to PEI FT-IR results of other studies [9, 18, 51].

Comparing the spectra of PEI and QA-PEI, it is possible to observe qualitative evidence of proper nitrogen alkylation through a weak quaternary ammonium absorption band around 967 cm-1 [9, 18, 51] that is absent on the pristine PEI FT-IR spectrum. Other bands on the QA-PEI spectrum arc a band around 3400 cm"1 (N-H), bands at 2956, 2920, 2860 and 1464 cm"1 (C-H), and at 1620 cm"1 (N-H) [9, 18, 51]. Results observed from FT-IR corroborate the results obtained from the elemental analysis performed on PEI and QA-PEI, indicating that the alkylation process successfully converted the amine groups on PEI into quaternary ammonium groups.

3.4. Ultraviolet (UV)-visible transmittances

PMMA is a remarkable material in its optical properties, achieving transparency values of around 92% [44, 52], making this polymer widely used in applications where this property is required. Therefore, formulations based on PMMA should not make optical behavior deviate much from what is already usual for optical applications.

Figure 6 shows the UV-visible transmittance curves for pristine PMMA, and the PMMA/QA-PEI blend films in the range from 200 up to 600 nm. It is observable that pristine PMMA reaches the highest transmittance levels, around 88% at 600 nm. All sample films were prepared with controlled thicknesses between 0.270 and 0.320 mm.

For the samples with 1, 2, and 5% QA-PEI concentrations, it is noticeable that transmittance values lowered to 76, 75, and 70% at 600 nm, respectively, showing a tendency for transmittance values to decrease as QA-PEI concentration raises, extending up to 10% QA-PEI concentration, with about 50% transmittance. These results indicate that, as QA-PEI content rises on the samples, the dispersed QA-PEI phase coalesces, lowering the PMMA/QA-PEI transparency due to a higher visible-light scattering by the larger QA-PEI droplets [44, 52-54]. This is supported by the SEM images (discussed below), which show evidence of the QA-PEI droplet size.

Adequate transmittance values arc still reachable even at 5% QA-PEI concentration, making these blends still suitable for applications with high transparency. An increase in the QA-PEI domain size as the QA-PEI content increases would also explain the accentuation of the yellowish hue and slightly cloudy appearance in the visual aspect of the films, as seen in Figure 7, which shows the photographs of the blend's films.

Figure 8 shows UV-visible absorbance graphs for the produced samples. Pristine PMMA exhibits results similar to PMMA absorbance reported in the literature [53, 54], showing absorptions at wavelengths below 300 nm.

From Figure 8, it is possible to see all the chemical structures absorb radiation between 320 and 500 nm, except for pristine PMMA, indicating that the QA-PEI chemical structure is responsible for these absorptions. Below 320 nm, it can be observed that absorption progressively raised from pristine PMMA to PMMA/10%QA-PEI, which could be attributed to the absorption by both PMMA and QA-PEI, as PMMA is known for absorbing under 300 nm [53, 54]. The growing absorptions from PMMA/1%QA-PEI up to PMMA/10%QA-PEI may be due to the absorptions of the larger QA-PEI domains as QA-PEI content increases on the blends.

3.5. Opacity

The opacity values for pristine PMMA and PMMA/ QA-PEI polymer blends are shown in Table 3.

The opacity values rise as the QA-PEI concentration increases on the produced blends. ANOVA (p < 0,05) and Tukey's range test indicated that there are significant differences between the sample's opacity average means, especially from pristine PMMA up to PMMA/1%QA-PEI and from PMMA/5%QA-PEI up to PMMA/10%QA-PEI. Incremental addition of QA-PEI to the PMMA matrix raised its opacity as probably QA-PEI aggregated into larger domains as QA-PEI content was raised from 1 to 10 wt% in the blends, which intensified the light scattering effect on the UV-Vis measurements [53, 54].

3.6. Scanning electron microcopies (SEM)

Morphology of the produced PMMA/QA-PEI blends was assessed via scanning electron microscopies (SEM) on the cryo-fracturcd cross-sections of the films of pristine PMMA, PMMA/2%QA-PEI, and PMMA/10%QA-PEI, which arc shown in Figure 9. There is notable phase separation in PMMA/QA-PEI blends, establishing droplet-matrix morphology. Some QA-PEI droplets were pulled out after cryofracturing, indicating low adhesion between phases [55]. In addition, as QA-PEI content increases, the QA-PEI droplets become larger, presenting a size of 0.92±0.30 pm in the PMMA/2%QA-PEI blend and 4.18±2.20 pm in the PMMA/10%QA-PEI blend. This explains the observed effect of greater visiblelight scattering by QA-PEI droplets, whose size is larger than the visible-light wavelength range.

3.7. Contact angle

Contact angle measurements were performed on pristine PMMA and PMMA/QA-PEI blends to estimate PMMA hydrophobicity as QA-PEI is added. Results are shown in Table 4.

It can be observed that pristine PMMA has the highest contact angle among the samples, as it is more hydrophobic than QA-PEI. It is also observable that the presence of QA-PEI lowered the samples' static water contact angle values, indicating the sample's wettability increased with the raising of QA-PEI concentration. Among the blend samples, the static water contact angle values are not significantly different, which means that raising the QA-PEI content on the sample does not necessarily imply an increase in the sample's wettability.

3.8. Differential scanning calorimetry (DSC)

DSC curves for QA-PEI, pristine PMMA, and PMMA/QA-PEI polymer blends are shown in Figure 10.

QA-PEI glass transition temperature (Ti) was estimated at -10 °C and pristine PMMA Tg at 116.5 °C, according to what is typically reported in the literature [14, 56]. According to Table 5, PMMA decreases when it is blended with QA-PEI, reaching 113.8°C for PMMA/1%QA-PEI, 108.0°C for PMMA/2%QA-PEI and 107.4 °C for PMMA/ 5%QA-PEI, yielding a 9 °C difference compared to pristine PMMA. For PMMA/10%QA-PEI, the same tendency is not observed, as is estimated at around 112.8 °C for this sample, as seen in Table 5. QA-PEI Tg was not efficiently detected via DSC in the PMMA/QA-PEI blends.

These changes indicate that QA-PEI had an effect on the PMMA Tg, and the reason for this may be associated with some level of chemical interaction between these polymers. The interaction between the polar carboxyl groups of PMMA and the positive charges of the quaternary ammonium groups of QA-PEI forms hydrogen bonds, which can lead to some level of partial miscibility, decreasing PMMA or due to some plasticizing effect over PMMA molecules [56, 57]. The inversion of the behavior observed on the PMMA/10%QA-PEI sample could be associated to some sort of saturation of the interactions between PMMA and QA-PEI chains, as QA-PEI content is raising on the samples, but it is not necessarily yielding more interactions with PMMA, which is suggested by the SEM images, which shows larger QA-PEI domains on the PMMA/ 10%QA-PEI sample, meaning that, beyond a certain concentration, QA-PEI tends to coalesce in itself, stagnating the available surface area for interacting with PMMA.

3.9. Thermogravimetric analyses (TGA)

The thermal stability of pristine PMMA and the produced blends was assessed through thermogravimetric analyses (TGA) and their TGA curves are shown in Figure 11. Pristine PMMA exhibits three subtle but different stages of weight loss: the first stage starts around 100 °C and goes up to 200 °C, and it is probably associated with depolymerization initiation of the degradation on the head-to-head bonds, which are less stable along the PMMA chain, and also to release of volatile materials, such as residual solvent molecules released during the glass transition as chains gain mobility [57, 58]. The second stage starts at around 220 to 265 °C. It is usually attributed to depolymerization initiated at the chain ends, especially at unsaturations created on chains terminated by disproportionation in polymerization [57, 58]. The third stage normally starts around 260 °C and goes to about 370 °C. This is attributed to random chain breaking of the main chains of the polymer [56-59]. The pristine PMMA produced in this study lost about 12% of its weight in the first stage, 15% of its weight in the second stage, and the rest of its mass in the third stage, as can be seen in Figure 11.

As can be observed from Figure 11, PMMA/QA-PEI blends show different decomposition behaviors compared to pristine PMMA. The second and third weight loss stages are overlapped on the TGA curves of these blends. It can also be noted that in the temperature range between 150 and 250 °C, the thermal resistance of the blends seems slightly lower than the resistance of pristine PMMA, but above 250 °C, this behavior reverses, and the thermal resistance of the blends rises in comparison with pristine PMMA as can be seen on Table 6.

It is possible to see an increase in the Tonset for the 1st stage of the weight losses for the produced blends when compared to the pristine PMMA, and also that Tonset rises as QA-PEI content rises on the blends. It is also observable that there was an increase in the Tonset for the 2nd stage of the produced blend samples when comparing with both the 2nd and 3rd stages Tonset of the pristine PMMA, which is indicative that QA-PEI increased the blend thermal resistance in comparison to pristine PMMA. It is also noticeable that the 2nd and 3rd weight loss stages overlapped for the produced PMMA/QA-PEI blends. These results may be due to the interactions with QA-PEI chains, which may have stabilized the PMMA chains, raising their resistance against both the depolymerization and the chain-breaking processes that occur during degradation [60, 61]. It should also be noted that thermal stability rises steadily as QA-PEI content increases on the PMMA/QA-PEI blends.

3.10. Mechanical properties

Tensile strength for pristine PMMA and all the blends are shown in Figure 12, and it is possible to observe that the tensile strength values are higher for the produced blends in comparison with pristine PMMA, but also that there are no significant differences among the blends samples themselves (ANOVA, p < 0.05).

Uncompatibilized polymer blends tend to have their mechanical resistances hindered due to phase separation, exhibiting mechanical failures at the interfaces of their component phases [62]. From Figure 12, it is possible to see that this was not the case for PMMA/ QA-PEI blends, which showed tensile strengths higher than pristine PMMA, probably due to some level of interactions between the carboxyl groups of PMMA and the positive charges of QA-PEI [62, 63]. There are no significant differences in tensile strength among the PMMA/QA-PEI blends, even though the QA-PEI content was raised, and that may be attributed to the limited miscibility of QA-PEI on PMMA, which restricted the interaction levels between QA-PEI and PMMA, thus limiting the reinforcing effect of QA-PEI over the PMMA chains [64, 65].

Figure 13 shows that the elongation at break does not vary significantly among the samples (ANOVA, p < 0.05), including pristine PMMA. PMMA is considered a brittle polymer [54], and QA-PEI did not alter the PMMA behavior.

Figure 14 shows that the tensile modulus is not significantly different among the samples (ANOVA, p < 0.05), including pristine PMMA. Just like the elongation at break, the Tensile Modulus was not affected by the presence of QA-PEI.

4. Conclusions

The alkylation process successfully converted the amine groups of PEI into quaternary ammonium groups, as shown by the elemental analyses, FT-IR, and ^-NMR.

The mixtures between PMMA and QA-PEI yield phase-separated polymeric blends, as shown by the SEM images. PMMA 7g decreased with the addition of QA-PEI, suggesting some level of chemical interactions with PMMA. Regarding the water static contact angle, the PMMA/QA-PEI blends are more hydrophilic than pristine PMMA.

In PMMA/QA-PEI blends, the QA-PEI droplet size affects the blend's optical properties, such as transmittance and opacity, when compared to pristine PMMA. However, these blends would still be suitable in applications where these optical properties are not critical while providing other properties and allowing applications characteristic of cationic polymers, such as drug delivery systems, purification, antifouling, and biocidal, which will be addressed in future studies.

TGA of the blends showed that the thermal stability of the blends improved in comparison with pristine PMMA, and assessment of mechanical properties showed that tensile strength was also improved with the addition of QA-PEI. At the same time, tensile modulus and elongation at break results were unaffected.

In summary, the proposed methodology successfully produced PMMA/QA-PEI blends aiming to modify PMMA properties and enable a new panorama of applications in the future.

Acknowledgements

The authors would like to acknowledge 'Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq' (Grant number 140443/2020-7) and 'Fundaçao de Amparo à Pesquisa do Estado de Sao Paulo- FAPESP' (Grant numbers 2021/09341-1 and 2023/12419-8) for the financial support. This research used facilities of the Brazilian Bioscience and Nanotechnology National Laboratories (LNBio and LNNano), parts of the Brazilian Centre for Research in Energy and Materials (CNPEM), a private non-profit organization under the supervision of the Brazilian Ministry for Science, Technology, and Innovations (MCTI). The LNBio staff is acknowledged for the assistance during the experiments (proposal numbers: 20233882 and 20233635). The authors declare that they have no known competing interests or personal relationships that could have influenced the work reported in this paper. In this work, Rafael Affonso Netto was responsible for conceptualization, methodology, investigation, data curation, and writing the original draft. Guilherme Ribeiro de Carvalho was responsible for methodology, investigation and review. Lucas Henrique Staffa was responsible for co-supervision, investigation, data curation, writing, review, and editing and Liliane Maria Ferrareso Lona was responsible for conceptualization, funding acquisition, supervision, writing, review, and editing.

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Received 29 August 2025; accepted in revised form 13 November 2024

Corresponding author, e-mail: [email protected] © BME-PT

References

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